Myopia (also called sort-sight) is a common ocular condition in which distant objects appear blurred whereas near objects are seen clearly. The prevalence of myopia, which is about 25% in developed countries and may be 70-80% in parts of Asia, has significant socioeconomic and public health consequences. Even people with relatively low degrees of myopia usually require an optical correction (eg spectacles or contact lenses) to allow them to drive a car or see the school blackboard, whereas those with high myopia also have an increased risk of developing blinding conditions such as retinal detachment and glaucoma. Myopia often develops during childhood and typically increases in severity (requiring progressively stronger spectacles to correct it) until early adulthood, although the final amount of myopia that develops will vary between individuals.
Myopia is generally characterised by an abnormal enlargement of the eye-ball which has the effect of moving the light-sensitive tissue (the retina in the back of the eye) out of the focal plane of the optical components of the eye. Thus, images of distant objects are brought to focus in front of the retina, rather than in the plane of the retina. Images of distant objects are therefore seen as blurred. In high levels of myopia, the marked enlargement of the eye-ball also results in a stretching of the retina and its associated blood supply, which renders the eye more susceptible to retinal detachment, glaucomatous damage and degenerative myopic retinopathy.
The aetiology of myopia is poorly understood. Both genetic and environmental factors have been implicated and in susceptible individuals myopia progression is thought to be associated with excessive near work (eg reading), possibly because the prolonged muscular effort of focussing the eyes at near (accommodation) results in a lag of accommodation (insufficient accommodation) and hyperopic retinal defocus. The correction of myopia requires minus-powered lenses which demand a greater accommodative effort for near work than is required without the lenses. This greater effort (and thus greater accommodative lag) has been implicated in exacerbating myopia progression and attempts have been made to reduce it by prescribing bifocal spectacles or Progressive Addition Lenses (PALs). Most evidence indicates that if bifocals/PALs slow myopia progression compared to conventional lenses, then it is only by a small amount. A variety of other methods have been used in attempts to slow myopia progression (eg vision therapy, distance under-correction and biofeedback training) but there is no convincing evidence that myopia progression is reduced by these procedures. In one recent study (Chung, Mohidin & O'Leary, Undercorrection of myopia enhances rather than inhibits myopia progression, Vision Research, 42 (2002) 2555-2559) 47 children had their myopia undercorrected by 0.75 D (which reduced their distance vision to 6/12). Over a period of two years, the progression of myopia in these children was shown to be significantly greater than in a control group of children wearing a full conventional correction. They concluded that myopic defocus speeds up myopia development in already myopic children. However, although bilateral under-correction results in myopic retinal defocus for distance viewing, clear retinal images in both eyes are experienced for near viewing. Use of pharmacological agents, in particular atropine eye-drops, to reduce myopia progression has been investigated in several studies and recent trials have provided evidence that repeated instillation of atropine may be effective in reducing myopia progression. However, myopia typically progresses over a number of years. The prospect of applying drug therapies long-term to large numbers of healthy children poses significant problems in relation to drug toxicity and other unwanted side effects.
Animal studies have demonstrated that the normal developing eye grows in a coordinated manner so that the length of the eye matches the power of its optical components, resulting in emmetropia (no refractive error). Furthermore, when a lens is placed in front of a developing animal's eye, the eye alters its growth in such a way as to compensate for the imposed defocus. Over time, the eye adjusts its axial dimensions to achieve functional emmetropia with the lens in place. Thus, eyes that have worn minus lenses (initially causing hyperopic retinal defocus, with images focused behind the retina) become elongated and are thus myopic on lens removal. Eyes that have worn plus lenses (causing myopic retinal defocus, with images focussed in front of the retina) are shortened and are thus hyperopic (long sighted) on lens removal. Such compensatory responses to defocusing lenses have been demonstrated in several species, ranging from chick to monkey.